Blender Cloud Login For 2.91
https://docs.blender.org/manual/en/latest/addons/system/blender_id.html
System: Blender ID authentication addon.
Search for ID in search and you will find it.
This is a doc loop that is unclear - a ticket should be put in place to mention that the addon referenced is an addon and not a website.
tickets can probably be put in to https://github.com/dfelinto/blender-cloud-addon